1. Technical Field
This invention relates to a phase lock loop circuit, and more particularly to a phase lock loop circuit for use where an incoming signal, which is to be tracked, periodically shifts in phase by 180 degrees.
2. Background Art
Phase lock loop (PLL) circuits are well known in the art. The circuit is used to produce a reference signal having a pre-determined frequency and phase relationship to an incoming signal. Normally, this incoming signal does not periodically invert in phase. However, some applications would benefit from such an incoming signal. For instance, data could be encoded onto the signal via a series of phase reversals. One such application is that of a digitizer. Digitizer systems for use as input devices to computers are also well known in the art. In a typical digitizer system, a pointing device is moved over the sensing (or working) surface of a tablet wherein the sensing surface of the tablet defines the boundaries of an X-Y coordinate system. The position of the pointing device on the sensing surface of the tablet is usually determined by an electrostatic or electromagnetic sensing link.
Recently, "cordless digitizers" in which there is no connecting cable between the pointing device and the tablet have become popular. In an electro-magnetic version as manufactured by the assignee of this application, a digitizer tablet employs grid conductors for each of the two coordinate directions. A pointing device is the "driven" member and emits an alternating current (AC) magnetic field from its tip at a given frequency. The magnetic field emanations induce signals into the grid wires which are then used by tablet electronics to determine the location of the pointing device. In practice, first the grid conductors for one coordinate direction are individually selected by a selector device and the induced signal therein is sensed. Then the grid conductors for the other coordinate direction are similarly selected and sensed. The amplitude characteristics of the induced signal and its magnitude are used by the tablet electronics and logic to determine how close the pointing device is to the grid conductor being sensed. When all the grid conductors have been sampled, the location of the pointing device on the tablet's sensing surface is derived. This position determination process typically requires that a reference signal, having a known frequency and phase relationship to the signal transmitted by the pointing device, be generated in the digitizer tablet. The reference signal is used to demodulate the induced signal. This is where the aforementioned PLL circuit comes into play. The PLL circuit is used to generate the reference signal.
To allow the user of the digitizer to indicate when positional information is being transmitted from the pointing device to the tablet, the pointing device usually has one or more manually-operated buttons associated with it. Typically, when the user wishes to transmit such information or data, one of the buttons is depressed. The pointing device then transmits an encoded signal to the digitizer tablet. The tablet electronics senses the encoded signal and performs whatever function is associated with that code.
Many different encoding schemes have been employed to transmit the aforementioned code in a cordless digitizer. Among these are Frequency Shift Keying (FSK), Phase Shift Keying (PSK), and Amplitude Modulation (AM), and variations thereof. These encoding schemes generally modify the basic pointing device signal in some way so as to create a binary code decodable by the digitizer electronics. As suggested above, one possible PSK encoding scheme might involve shifting the phase of the signal transmitted by the pointing device by 180 degrees, at known intervals, to represent data bits. For instance, if the phase in the pointing device signal is unchanged from the last known interval, this no-change condition could represent one binary state. If the phase had changed, however, the condition would then represent the other binary state. Accordingly, a binary encoded message may be transferred from the pointing device to the tablet for decoding by the digitizer logic circuits via this phase change method. Of course, this encoding method need not be limited to binary encoding. For instance, pre-determined patterns of phase reversals could be used to represent a plurality of different informational packets.
However, a problem in employing a phase inverting signal arises. A typical phase lock loop circuit responsible for generating the required reference signal would become unstable during the times it attempts to match phase reversals in the reference signal. This instability is caused when a voltage controlled oscillator (VCO), which is normally a part of the circuit, is driven to invert the phase of its output to match that of the pointing device's signal. During the time it takes for this phase change to occur, the reference signal will not be at a known phase relationship to the pointing device's signal. Therefore, it can not be used for its intended demodulation purposes.
It is, therefore, an object of the present invention to provide the architecture for a phase lock loop circuit whose output remains stable and closely tracks an incoming signal, even when the incoming signal is shifted in phase by 180 degrees.
It is another object of the present invention to provide the architecture for a phase lock loop circuit which outputs a signal indicating when a phase reversal has occurred in the incoming signal.
It is yet another object of the present invention to provide the architecture for a phase lock loop circuit to be used in a digitizer which outputs a signal indicating that data is being transmitted via a series of phase reversals in the incoming signal, instead of positional information.
In addition, other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.